The present disclosure is directed to devices and systems for producing purified silicon, and the resultant purified silicon.
Extreme purity in semiconductor silicon is necessary, and best available silicon purity is prized in semiconductor production. Improved silicon purity translates into improved semiconductor production statistics and improved semiconductor operation. Silicon based photocell efficiency and photocell production could also be more efficient if much purer silicon was available cost effectively for silicon based photocell production. In general, given ideal processing, the ideal silicon for any photocell or semiconductor purpose (setting cost aside) would be purity so high that no elements other than silicon were detectable in the silicon, purity so high that the exact value of the concentrations of impurities were immaterial for any practical purpose, and in the ideal limit, perfect or asymptotic purity, where the presence of single atoms of impurity elements in the silicon was improbable. Such “perfect for all practical purposes” purity would provide maximum control of crystallization, P—N doping, resistivity, and charge carrier lifetime. Every impurity element in the Periodic Table slows, degrades and disorders silicon crystallization to some degree. Elements in electron acceptor group 13 (B, Al, Ga, etc) and electron donor group 15 (N, P, As, Sb etc) determine whether the dominant charge carrier in the silicon is electrons or holes, and are very important for resistivity. The transition metals all reduce charge carrier lifetimes, with the deleterious effect on charge carrier lifetime increasing moving to the left and down on the periodic table. Ta, Mo, Mb, Zr, W, Ti, and V atoms significantly reduce crystalline silicon charge carrier lifetimes at sub part per billion concentrations. For the best possible photocells, or the best semiconductors (especially for high frequency operation, or where energy costs are important, as in cell phones and tablets) one wants the longest available charge carrier life and the highest available silicon resistivity.
The energy costs of silicon production are also important. Energy costs are especially important for silicon photocell production if silicon photocells are to have a practical chance of replacing fossil fuel energy production on a world scale. Current photocell energy production (which is mostly from silicon based photocells) is less than 1% of world energy consumption. At that relatively small scale, the energy cost of refining the silicon may not be crucial. However, if energy production from silicon photocells is ever to approach world energy consumption, so as to significantly displace fossil fuel production, it would be desirable to greatly reduce the energy cost of refined silicon production—and desirable to do so with much higher purities than are available in “solar grade silicon” now.
It is estimated that the energy cost to manufacture 1 kg of silicon wafers for photocells now is about 1000 kilowatt hours. Energy payback for current production silicon photocells is therefore more than two years. Very rapid growth of multi-terawatt scale silicon photocell deployments needed to significantly replace world fossil fuels use with this silicon energy cost would require inconvenient and probably commercially impossible energy inputs. The scale of silicon production needed to produce multi-terawatt outputs of silicon based photocells is large. Assuming that 500 watts of photocells can be made per kilogram of purified silicon, production of 1012 watts of photocells in ten years of continuous production would take production of about 550 tons of purified silicon per day during that decade of production. To replace world consumption of fossil fuels with photocells, something around fifty times this production would be needed—or something around 13,750 tons/day of purified silicon every day for twenty years. For this production to be feasible in terms of energy paybacks, it would be desirable to cut current art silicon purification energy costs by a factor of ten or much more.
Looking at thermodynamic limits, the theoretical energy requirement of silicon purification and wafer fabrication is something less than twice the energy requirement to melt the silicon—something less than 1.5 kilowatt hour per kilogram of silicon, or less than 1% of current energy costs. But the actual history of incremental development has not involved any substantial, sustained effort to converge on these low energy costs, which would involve committing to technology fundamentally different from established patterns. Historical efforts to produce “solar grade silicon” have involved variations on a theme with inherently high energy costs. Proceeding according to this theme, silicon is oxidized to a volatile (SiCl4; SiHCl3; SiI4; SiH4, SiHCl3; SinF2n+2; SiHBr3; or SiF4) and that volatile is multiply distilled. The purified volatile is then reduced to solid silicon in a reactor. All of these approaches involve energy costs far higher than the thermodynamic limit. The basic silicon volatile distillation and reduction pattern locks in energy costs and capital costs that do not constrain current relatively small scale photocell deployments, but the energy costs of this established pattern rule out the enormous production scales necessary for photocells as a full solution to the world's energy scarcity and global warming problems, where production of tens of terawatts of photocells will be necessary, and will have to be produced within a relatively few years.
If photocells are to practically replace fossil fuels, one of the technical requirements will be a process for purifying silicon with much lower (ideally arguably minimum possible) energy costs, much lower (arguably minimum possible) operating costs, and the capacity for high production rates (up to millions of kg/day). The highest possible silicon purity would be desirable for this process, ideally purity much higher than any available today, to facilitate the maximization of silicon photocell efficiency and to facilitate the minimization of silicon photocell production cost.
Setting the issue of photocell production aside, there is also a significant and ongoing market need to improve the purity-cost tradeoff for semiconductor silicon, and for silicon supplied for metal casting and silicone feedstock production purposes as well.